For the Allies, all communication between their northern and southern forces was severed by the arc of the westward German advance from the Ardennes to the Somme. The Allied armies in the north, having fallen back from the Dyle Line to the Escaut (Schelde), were being encircled, and already on May 19 the British commander, Viscount Gort, was considering the withdrawal of the BEF by sea. On May 21, however, to satisfy orders from London for more positive action, he launched an attack from Arras southward against the right flank of the Germans’ corridor; but, though it momentarily alarmed the German high command, this small counterstroke lacked the armoured strength necessary for success. Meanwhile, Guderian’s tanks had swept up past Boulogne and Calais and were crossing the canal defense line close to Dunkirk when, on May 24, an inexplicable order from Hitler not only stopped their advance but actually called them back to the canal line just as Guderian was expecting to drive into Dunkirk.

Dunkirk was now the only port left available for the withdrawal of the mass of the BEF from Europe, and the British Cabinet at last decided to save what could be saved. The British Admiralty had been collecting every kind of small craft it could find to help in removing the troops, and the British retreat to the coast now became a race to evacuate the troops before the Germans could occupy Dunkirk. Evacuation began on May 26 and became still more urgent the next day, when the Belgians, their right wing and their centre broken by Reichenau’s advance, sued for an armistice. On May 27, likewise, bombing by the Luftwaffe put the harbour of Dunkirk out of use, so that many of the thousands of men thronging the 10-mile stretch of beaches had to be ferried out to sea by petty craft pressed into service by the Royal Navy and manned largely by amateur seamen, though the harbour’s damaged breakwater still offered a practicable exit for the majority. By June 4, when the operation came to an end, 198,000 British and 140,000 French and Belgian troops had been saved; but virtually all of their heavy equipment had to be abandoned, and, of the 41 destroyers participating, six were sunk and 19 others damaged. The men who were saved represented a considerable part of the experienced troops possessed by Great Britain and were an inestimable gain to the Allies. The success of the near-miraculous evacuation from Dunkirk was due, on the one hand, to fighter cover by the Royal Air Force from the English coast and on the other to Hitler’s fatal order of May 24 halting Guderian. That order had been made for several reasons, chiefly: Hermann Göring, head of the Luftwaffe, had mistakenly assured Hitler that his aircraft alone could destroy the Allied troops trapped on the beaches at Dunkirk; and Hitler himself seems to have believed that Great Britain might accept peace terms more readily if its armies were not constrained into humiliating surrender. Three days passed before Walther von Brauchitsch, the German Army commander in chief, was able to persuade Hitler to withdraw his orders and allow the German armoured forces to advance on Dunkirk. But they met stronger opposition from the British, who had had time to solidify their defenses, and almost immediately Hitler stopped the German armoured forces again, ordering them instead to move south and prepare for the attack on the Somme–Aisne line.

The campaign in northern France was wound up by Küchler’s forces, after both Guderian and Reichenau had been ordered southward. Altogether, the Germans had taken more than 1,000,000 prisoners in three weeks, at a cost of 60,000 casualties. Some 220,000 Allied troops, however, were rescued by British ships from France’s northwestern ports (Cherbourg, Saint-Malo, Brest, and Saint-Nazaire), thus bringing the total of Allied troops evacuated to about 558,000.

There remained the French armies south of the Germans’ Somme–Aisne front. The French had lost 30 divisions in the campaign so far. Weygand still managed to muster 49 divisions, apart from the 17 left to hold the Maginot Line, but against him the Germans had 130 infantry divisions as well as their 10 divisions of tanks. The Germans, after redisposing their units, began a new offensive on June 5 from their positions on the Somme. The French resisted stiffly for two days, but on June 7 the German tanks in the westernmost sector, led by Major General Erwin Rommel, broke through toward Rouen, and on June 9 they were over the Seine. On June 9 the Germans attacked on the Aisne: the infantry forced the crossings, and then Guderian’s armour drove through the breach toward Châlons-sur-Marne before turning eastward for the Swiss frontier, thus isolating all the French forces still holding the Maginot Line.